This invention relates to an improved differential pressure indicator for hydraulic fluid filters and the like and more particularly to a magnetically operated pop up type indicator.
In the past, many different forms of magnetic pressure indicators have been used for the sensing of differential fluid pressure of a particular value in a fluid system and for providing a visual indication of this condition. Many of these devices are of simplified construction suited for application in existing filter structures and the like.
One example of prior art device is shown in U.S. Pat. No. 2,942,572 wherein magnet elements are housed in a piston responsive to fluid pressure and in an indicator mechanism magnetically coupled to the piston. This prior art showing is typical of many of these devices in that the magnets are oriented in a common direction so that an attraction therebetween takes place, typically to retain the indicator in a recessed position. As differential fluid pressure increases due to the clogging of a filter element or the like, the piston is moved downwardly against the bias of a coil spring and allows the indicator to be released and to be moved to a visible position by means of the bias exerted by a second coil spring. In this apparatus a bimetal strip is employed to prevent movement of the indicator at low temperature levels when, due to the high viscosity of the fluid, transient excessive differential fluid pressures might be realized.
A similar form of device is shown in U.S. Pat. No. 3,442,248 wherein a simplified button structure is housed in a threaded body member suited for placement in a typical fluid filter or the like. In this apparatus the indicator button is retained in a retracted position by the force of a fixed permanent magnet in the body which resists the bias of a coil spring urging the indicator button outwardly. In this apparatus the piston is formed also of magnetically attractable material and when it is moved to a position adjacent the magnet due to an increase in fluid pressure acting thereon, the piston will draw off sufficient magnetic flux such that the button spring will overcome the magnetic holding force and cause the indicator to pop outwardly of the body. Also in this apparatus a thermal assembly, comprising a bimetal strip, is employed to prevent movement of the indicator button at low temperature levels.
Another similar form of indicator is shown in U.S. Pat. No. 3,595,200 where again a fixed permanent magnet is employed to retain an indicator button in the retracted position against the bias of a coil spring. Several techniques are shown in this reference for moving a piston or similar structure adjacent the permanent magnet to draw off part of the magnetic flux and allow the indicator button to be released in a manner similar to that previously described.
A still further form of differential pressure indicator is shown in U.S. Pat. No. 4,029,042. Here a pair of magnets are used, one being fixedly attached at the top of the housing and the second affixed to the lower portion of an indicator button. The magnets are arranged in a common direction to provide attractive force therebetween. However, the indicator button is retained in the lower portion of its housing against an inner wall therein, being attracted by a piston formed of magnetic material. As the piston is moved downwardly due to an increase in fluid pressure the lower magnet will lose its keeper mechanism and will be attracted by the fixed upper magnet to move the indicator to an exposed position. This apparatus also employs a bimetallic element which contracts or expands at different temperature levels to allow or prevent movement of the indicator button.
In many of the prior art devices means such as the bimetal strip are often employed to prevent false indications of the pop up type indicator. These false indications are due not only to high viscosity fluid conditions but are often occasioned by vibratory conditions or the like where forces can cause the indicator to be released from its latching mechanism. The types of prior art devices which are activated by coil springs or which rely on magnetic attractive forces are particularly susceptible to these vibration problems inasmuch as once released from a stable position an automatic movement to the extended visual position occurs. Typically, in these devices the stable position is somewhat sensitive in that a close balance must be achieved between operating spring forces, magnetic attraction forces and the like.